staging: comedi: pcl816: rename 'irq_blocked' in private data

This member in the private data is a flag that indicates that an analog
input async command is currently running. Rename it to make this clear.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>